With the increasing awareness of pollution and, in particular, soil contamination, considerable effort is being devoted to soil clean-up. Whilst clean-up of soils contaminated with volatile organics/hydrocarbons has been successfully practiced for the last 10 years, using a variety of methods, much less progress has been done in cleaning the soils contaminated with tarry/oily highly viscous organics/hydrocarbons containing also slag, carbonaceous materials (like coal, char, coke) and other inorganic impurities. A notable exception is a soil washing technique where coal is employed as contaminant adsorbent; this technology is described in U.S. Pat. No. 5,019,245, issued May 28, 1991 and U.S. patent application Ser. No. 08/065,489 (now abandoned) filed May 24, 1993 and it is known and referred to as Clean soil Process.
While the technology described in the Patent and Patent Application referred to, above, can effectively separate a heterogeneous tar/oil contaminated soil into homogeneous product streams, the cleanliness of the two streams of processed soil, namely "coarse clean solids" (like rocks, pebbles, gravel, etc.) and the "fine clean soil" (mineral matter components; top size 3.3 mm, preferably 1.0 mm) is not always satisfactory. Typically the coarse clean solids will contain about 100-7,000 ppm of total toluene soluble material and the concentration of PAH's in these solids can vary from about 10-300 ppm. The fine clean soil will contain about 100-2,000 ppm of total toluene soluble material and the concentration of PAH's in fine soil can vary over a range of 1-100 ppm.
The residual concentration of contaminants in clean coarse and fine soils generated by the method described in the Patent and Patent Application referred to above is, in some cases, too high to return these soils to the ground. The soil precleaned by Clean Soil Process could be subjected to post-treatment by employing some suitable post-treatment method but this would, as a rule, significantly add to the total clean-up cost.
It is the objective of the present invention to provide a process for cleaning the coarse and fine soils to a very high degree of cleanliness, namely: the residual concentration of total toluene soluble material to 100 ppm or less and the residual concentration of total PAH's below 3 ppm.
It is yet another objective of the present invention to achieve this high degree of cleanliness at a cost significantly lower than the combined cost of primary clean-up (using the Clean Soil Process) and subsequent post-treatment (using the conventional thermal desorption technology).
These and other objectives of the present invention will be apparent from the following description and from the practice of the invention.